Integrated arrays of modulators and lasers on electronics

ABSTRACT

A unit has an array of lasers having an emission surface through which beams can be emitted in a substantially vertical direction so as to define an emission side, drive electronics connected to a side opposite to the emission side of the array of lasers, and an array of modulators, located on the emission side of the array of lasers and connected to the drive electronics.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority under 35 USC 119(e)(1) of U.S.Provisional Patent Application Serial No. 60/365,998 and U.S.Provisional Patent Application Serial No. 60/366,032, both filed Mar.19, 2002.

[0002] This application is a continuation-in-part of commonly assignedU.S. patent application Ser. No. 09/896,189, U.S. patent applicationSer. No. 09/897,160, U.S. patent application Ser. No. 09/896,983, U.S.patent application Ser. No. 09/897,158 and U.S. patent application Ser.No. 09/896,665, all filed Jun. 29, 2001.

FIELD OF THE INVENTION

[0003] This invention relates to optical devices and, more particularly,to optical devices involving lasers and modulators.

BACKGROUND

[0004] External modulation has been used in some semiconductor lasersystems where one or a few edge emitting lasers were used in a lineararrangement. Single channel use of modulators with edge emitting lasersin a package is also therefore possible. However, presently, twodimensional arrays of edge emitting semiconductor lasers are not in theprior art, hence integration of modulators with such lasers is notpossible in the prior art. In addition, modulators that can beintegrated with arrays of vertical emitting format lasers, particularly,vertical emitting cavity, distributed feedback (DFB) lasers anddistributed Bragg reflector (DBR) lasers, are not available in the priorart.

SUMMARY OF THE INVENTION

[0005] As shown in FIG. 1, we integrate arrays of vertical cavitymodulators 100 on top of arrays of lasers 102 which are integrated ontop of electronics 104 (one or more chips). By employing the teachingsof the invention, we create large laser arrays in which each of thelasers can be operated at constant light output and the output can beswitched on an off, very rapidly, via external modulation throughintegration of the modulator array on top of the laser array. As aresult, we can accomplish a number of advantages including switchinglasers in a two-dimensional semiconductor laser array at rates bothbelow, and in excess of, 10 Gb/s.

[0006] The advantages and features described herein are a few of themany advantages and features available from representative embodimentsand are presented only to assist in understanding the invention. Itshould be understood that they are not to be considered limitations onthe invention as defined by the claims, or limitations on equivalents tothe claims. For instance, some of these advantages are mutuallycontradictory, in that they cannot be simultaneously present in a singleembodiment. Similarly, some advantages are applicable to one aspect ofthe invention, and inapplicable to others. Thus, this summary offeatures and advantages should not be considered dispositive indetermining equivalence. Additional features and advantages of theinvention will become apparent in the following description, from thedrawings, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007]FIG. 1 is a simplified representation of an array of modulators ontop of an array of lasers on top of an electronic integrated circuit;

[0008]FIG. 2 shows a modulator having connections formed for mating withconnections on an example integrated laser device previously integratedwith an electronic chip;

[0009]FIG. 3 shows an alternative modulator having connections formedfor mating with connections on a laser device using a back side process;

[0010]FIG. 4 shows a modulator unit for use with discrete redundantlasers or a single laser having redundant active regions;

[0011]FIG. 5A shows an edge-emitting DFB of the prior art;

[0012]FIG. 5B shows an edge-emitting DBR of the prior art;

[0013]FIG. 6 shows the parallel transceivers created by integrating thedevices, one-at-a-time, in a one-dimensional (i.e. linear) array;

[0014]FIG. 7A shows an upwards or “top” emitting grating coupled laser;

[0015]FIG. 7B shows a downwards or “bottom” emitting grating coupledlaser;

[0016]FIG. 7C shows an upwards or “top” emitting micromirror coupledlaser;

[0017]FIG. 7D shows a downwards of “bottom” emitting micromirror coupledlaser;

[0018]FIG. 8 shows a two-dimensional array and integrated intimatelywith drive electronics present in the integrated circuit on which theywere mounted; and

[0019]FIG. 9 shows a side view of portion of an integrated unit made upof a two dimensional array of grating coupled DBR lasers integrated withmodulators and an electronic integrated circuit by applying theteachings of the invention.

DETAILED DESCRIPTION

[0020] Commonly assigned U.S. patent application Ser. Nos. 09/896,189,09/897,160, 09/896,983, 09/897,158 and 09/896,665 and U.S. ProvisionalPatent Application Serial Nos. 60/365,998 and 60/366,032, are allincorporated herein by reference in their entirety, and describedifferent ways for integrating optical devices, including, but notlimited to, vertical cavity surface emitting lasers (VCSELs),distributed feedback (DFB) lasers and distributed Bragg reflector (DBR)lasers, with electronics, irrespective of whether they are top-side orbottom/back-side emitting, to form large optical device arrays and forcreating modules incorporating the resulting integrated opto-electronicchips.

[0021] At high data rates, for example, about 10 Gb/s or more, itbecomes difficult to turn lasers on and off rapidly enough to accomplishdata transmission by direct modulation of the lasers. This is becausewhen directly modulating a laser, electrons must be fed into the laserdevice in order to create light and then pulled out of the laser deviceto turn off the light during switching. However, this process requires aminimum amount of time and power to execute, due to fundamental physicalproperties of the laser, such as the capacitance of the device and thedecay time constant of electrons in the laser's active region.

[0022] As speeds become higher, the required rate of switchingapproaches, and then eventually exceeds, the minimum time required to doso. As a result, as speeds increase, the process becomes extremelydifficult (and eventually impossible) to perform.

[0023] As a result, at higher speeds, optical data transmission is oftenaccomplished using what is called “external modulation” in which alasers remains continuously “on” and a device external to the lasercontrols the light output. In one type of external modulation, a deviceknown as a “modulator” is placed between the output portion of the laserand the external world. The modulator turns the light on and off therebycreating the effect of turning the laser on and off. A modulator canturn on and off the light, relative to the external world, via a numberof mechanisms. For example, a modulator can be transparent in one stateand absorptive in another, a modulator can be transparent in one stateand reflective in another, a modulator can use changes in refractiveindex to shift the resonance wavelength of the laser so that the cavityresonant wavelength is no longer on the gain region, or a modulator canbe irrelevant to and then disruptive to the optical properties of thelaser (for example by changing the effective reflectivity of one of thelaser's mirrors in a switchable fashion).

[0024] Our aforementioned incorporated applications, described processesthat can be also be used to integrate a large array of modulators withan array of active optical devices (e.g. lasers and/or detectors) on anelectronic chip.

[0025] We specifically describe herein some modulator specific aspectsof the previous approaches to ensure that modulators are made compatiblewith the vertically emitting lasers described therein and to ensure themodulators are made electrical contact compatible with an electronicchip already containing lasers.

[0026]FIGS. 2 through 4 show example formats for integration of anindividual modulator in an array of modulators, it being understood thatthe process can be preformed in the same manner on an individual deviceor wafer scale basis.

[0027]FIG. 2 shows a modulator 200 having connections 202, 204 formedfor mating with connections 206, 208 on an example integrated laserdevice 210 previously integrated with an electronic chip 212, in thiscase a VCSEL. In FIG. 2, the connections for the modulator were madeusing one of the incorporated by reference topside processes. In thelower half of FIG. 2 the devices after the contacts have mated and beenbonded.

[0028]FIG. 3 shows an alternative modulator 300 having connections 302,304 formed for mating with connections 206, 208 on a laser device 210 ofFIG. 2, using a back side process of an application incorporated hereinby reference, and, in the lower half of the figure, the devices afterthe contacts have mated and been bonded.

[0029]FIG. 4 shows a modulator unit 400, for use with discrete redundantlasers 402, 404 or a single laser having redundant active regions (notshown). In this configuration, separate standoffs 406, 408 are used tophysically elevate the contacts 410 of the electronic chip 412 to wherethe modulator unit's contacts 414 can mate with the elevated contacts416. Th lower half of the figure shows the modulator unit 400 after ithas been located above the lasers 402, 404 integrated with theelectronic chip 412. In addition, as shown in the lower half of FIG. 4;a lens 418 is mounted on top of the modulator to allow coupling ofemitted beams from the lasers with a common fiber or other element (notshown).

[0030] As noted in the applications incorporated by reference,preparation and integration processes are slightly different dependingupon whether the lasers (or detectors) and modulators are optimized forlight emitting from/entering in from the top of the respective devicesor the bottom (i.e. backside) of the respective devices, whether or notthrough a substrate.

[0031] Three representative alternative implementations, createdaccording to the processes of the incorporated by reference applicationsprovide for the interconnection between a modulator and a laser. Theserepresentative implementations are shown in FIGS. 2 through 4, althoughother implementations can be made in addition to those shown hereinthrough application of those techniques.

[0032] The basic approach comprises:

[0033] Integrating lasers in a large array onto an electronic chip. Asdescribed in the incorporated by reference applications, this is doneusing either a) bottom emitting lasers, where a funnel or other openingis etched into the substrate, the substrate is partially or completelyremoved to allow optical access, or with no substrate removal, if thesubstrate is optically transparent at the laser's wavelength, or b) topemitting lasers (i.e. emission of the laser is not towards thesubstrate).

[0034] Ensuring that the lasers have “pass-through contacts” which allowelectrical connection to be made between the top of the laser waferpiece and the bottom of the modulator piece via contacts on the top ofthe laser piece and bottom of the modulator piece, without theelectrical contacts impacting the performance of the laser itself. Inother words, the contacts allow the modulators to be electricallyconnected to the electronic wafer through the intervening laser wafer).

[0035] Creating an array of modulators which allow optical access intoand out of the device and have all their electrical contacts on the sameside of the wafer piece that will be closest to the lasers and arrangedin a configuration that matches with the configuration of the“pass-through contacts” on the laser piece.

[0036] Integrating the modulators with the laser wafer piece in asimilar way to the way lasers are integrated onto the electronic chip inthe array.

[0037] A number of schemes for creating the passthrough regions and theformatting of the optical devices (either modulators or lasers) aredescribed in the applications incorporated by reference and as describedin the commonly assigned U.S. Provisional Patent Application Serial No.60/365,998 entitled “Topside Active Optical Device Apparatus AndMethod”, filed Mar. 19, 2002, the entirety of which is incorporatedherein by reference.

[0038] It should be understood that the modulators can also have asubstrate attached to them. Depending upon the particularimplementation, a funnel or other opening can therefor be made using oneof our techniques (or some other technique) in the substrate, thesubstrate can be thinned, or the substrate can be left alone if it isoptically transparent to the laser below. Moreover, depending upon theparticular modulator and its substrate location (i.e. top or backside),the funnel or other opening can be directed toward the lasers or towardthe outside world.

[0039] While others had proposed using DFB lasers in paralleltransceivers, they have typically been thought of and used strictly asedge-emitting devices (i.e. outputting parallel to the plane of thewafer). FIG. 5A shows an edge-emitting DFB of the prior art and FIG. 5Bshows an edge emitting DBR of the prior art. Thus, the paralleltransceivers using these edge emitting devices have been created byintegrating the devices, one-at-a-time, in a one-dimensional (i.e.linear) array. This is shown in FIG. 6. In FIG. 6, a circuit board 600has the individual edge emitting array of lasers 602 mounted along itsedge 604. An integrated circuit chip 606, containing the driver circuitsfor the lasers 602, is connected to the lasers 602 by a series ofwirebonds 608 via the circuit board 600.

[0040] Since, as noted above, our techniques are usable with any upwardsor downwards emitting (or receiving) devices, we recognized that ourinvention was also usable with grating coupled or angled micromirrorcoupled DFBs or DBRs, for example, because the grating or micromirrorcause their emitted beams to travel perpendicular to the plane of thewafer, such “top” or “bottom” emitting devices having been created byothers in the prior art to facilitate on-wafer testing of those devices.FIG. 7A shows an upwards or “top” emitting grating coupled laser of theprior art and FIG. 7B shows a downwards or “bottom” emitting gratingcoupled laser of the prior art. Similarly, FIG. 7C shows an upwards or“top” emitting micromirror coupled laser of the prior art and FIG. 7Dshows a downwards or “bottom” emitting micromirror coupled laser of theprior art.

[0041] As a result, our approach made it possible for thoseperpendicular emitting DFBs or DBRs to be configured in atwo-dimensional array and integrated intimately with drive electronicspresent in the integrated circuit on which they were mounted, such asshown in FIG. 8.

[0042] Advantageously, as we described, the integration of modulatorswith lasers is equally valid for a variety of lasers, provided that thelight from those lasers eventually emits vertically.

[0043] Thus, for example, a surface emitting DFB or DBR, such as agrating coupled DFB or a DBR coupled with a mirror, can be used to equal(or greater) effect and/or advantage than achievable with VCSELs.

[0044] As described in the incorporated by reference applicationsreferred to above, by applying the teachings of our inventions asdescribed therein, large-format two-dimensional transmitter ortransceiver arrays containing VCSEL, DFB or DBR lasers and electronicscan be made.

[0045] While VCSELs have the advantage of being a more mature andavailable technology, VCSELs have certain limitations. The output powerthey can provide is limited. The maturity of longer wavelength VCSELs,for example, at wavelengths of 1.3 microns and beyond, is less than atshorter wavelengths, and the chirp parameter (characterized by thewavelength spread or the change in phase of the output wave duringswitching) tends to be high. These characteristics limit the usefulnessof VCSELs for longer distance data transmission at ultra-high speeds.DFB lasers have superior characteristics which allow longer distance,high-speed data transmission. Accordingly, for long distance datatransmission at speeds in excess of 10 Gb/s, DFB lasers are superior toVCSELs.

[0046] In contrast to using VCSELs, using our approach with DFBs or DBRsallows large numbers of high-power, narrower linewidth, low chirp, longwavelength lasers to be integrated together on an electronic chip. Bydoing so, extremely large bandwidth can be achieved (relative to thebandwidth achievable with VCSELs) because resistive losses and/orcapacitive slowdowns are minimized. Thus, highly parallel (whether inspace or wavelength), low cost, intelligent transmitters or transceiverscan be made that can send data over several tens of kilometers which isfarther than can be done with other laser technologies, such as VCSELs.

[0047] However, as with VCSELs, the desire for ever faster transmissionrates means that, at some point, the time the DFB laser needs in orderto switch will become longer than the data bit rate transmission timeperiod. Thus, we can also straightforwardly create large arrays of DFBlasers with modulators closely integrated on top to achieve furtherbenefits to those obtained using VCSELs.

[0048] By way of brief overview, the process for integrating the DFBs orDBRs with the electronics is the same as described for the specificVCSEL examples in the incorporated by reference applications. Theprocess starts with a laser wafer or wafer piece containing largenumbers of surface emitting DFBs or DBRs. These lasers are surfaceemitting because they have an element near their output, for example agrating or micromirror to couple the light perpendicular to the surfaceof the wafer (either away from the substrate or into the substrate).

[0049] Then, depending upon whether the device is configured to emitaway from the substrate (i.e. “top” emitting) or towards it (i.e.“bottom” emitting) the appropriate process is performed as described indetail in the applications incorporated by reference and reiterated inbrief below.

[0050] If the devices are topside emitting devices: the laser wafer isattached to a carrier; if necessary, the laser wafer substrate isthinned; contacts are pattern etched on the back side of laser wafer ina pattern such that they will match the contacts of the electronic wafercontaining the drive circuits; if desired, an encapsulant is optionallyflowed between the laser and electronic wafers; and the laser andelectronic wafers are attached to each other at the contacts.

[0051] If the devices are bottom emitting devices, contacts are patternetched on the top side of laser wafer so as to match the contacts of theelectronic wafer containing the drive circuits; and the laser wafer isthen attached to the electronic wafer. Optionally, as with the topsideemitting device process, an encapsulant can be flowed between the wafersand/or the laser substrate can be thinned if necessary or desired.

[0052] These processes result in integrated devices which contain thevarious grating coupled DFB lasers integrated with electronics. Althoughthe approach can be performed without them, consistent withsemiconductor creation and etching techniques, making theseedge-emitting-type laser structures so that they contain etching layersand etch stop layers to help with fabrication of the contact layers and,potentially with the substrate thinning process is beneficial forcommercial scale production.

[0053]FIG. 8 shows, in summary fashion, the process steps and the endresult of the process, which serves as the starting point for thesimilar process of integrating modulators or detectors on top of thelasers in the array.

[0054]FIG. 9 shows a side view of portion of an integrated unit 900 madeup of a two dimensional array of grating coupled DBR lasers 902integrated with modulators 904 and an electronic integrated circuit 906by applying the teachings of the invention described in the incorporatedapplications and herein.

[0055] In another variant created according to the teachings of theinvention, the modulators can be replaced by detectors that are mostlyoptically transparent to the laser wavelength and integrated on top ofthe lasers in a similar manner. Alternatively, the modulators can beoperated as detectors to absorb an amount of light from the lasers asthe light passes through them. In other variants, both a modulator anddetector or two modulators can be stacked above the laser using the sameprocedures.

[0056] In those cases, the detector or modulator would then provide asampling of the output power from the lasers. The absorbed light willgenerate a current which can be routed to the integrated circuit andmeasured. If the power of the laser changes, for example, due totemperature changes or degradation of the laser over time, the absorbedcurrent will change proportionally. As a result, the electronic chipcould inject more current into the laser to keep the output powerconstant or, if redundancy is provided and a laser either could not putout enough power (i.e. it was effectively dead) or died, a backup lasercould be switched on its place.

[0057] Depending upon the particular implementation and needs, the fixeddetector would be used to absorb a small amount of light and let thelarge majority of light through, in which case the laser would bedirectly modulated.

[0058] In other implementations, the laser is externally modulated, byusing the upper device as a modulator and then using a lower modulatorto sample the output power by absorbing some of the light, either in the“on” state or the “off” state, or some weighted average of both, forexample, where a modulator is transmissive in the “on” state and absorbsin the “off” state; this means the modulator will let light through inthe “on” state and block light in the “off” state. However, in actualoperation, some amount of light will normally be absorbed, even in the“on” state due to the laws of physics and properties of the devices. Asa result, in actual operation, the generated current in the “on” or“off” state, or some average can be used, depending upon which worksbest in the particular design.

[0059] In still other implementations, the devices are arranged in astack in different orders, for example: a) electronics, detectors,lasers, modulators; b) electronics, lasers, detectors, modulations; orc) electronics, lasers, modulators, detectors.

[0060] It should be understood that the above description is onlyrepresentative of illustrative embodiments. For the convenience of thereader, the above description has focused on a representative sample ofall possible embodiments, a sample that teaches the principles of theinvention. The description has not attempted to exhaustively enumerateall possible variations. That alternate embodiments may not have beenpresented for a specific portion of the invention, or that furtherundescribed alternate embodiments may be available for a portion, is notto be considered a disclaimer of those alternate embodiments. One ofordinary skill will appreciate that many of those undescribedembodiments incorporate the same principles of the invention and othersare equivalent.

1. A unit comprising: an array of lasers having an emission surfacethrough which beams can be emitted in a substantially vertical directionso as to define an emission side; drive electronics connected to a sideopposite to the emission side of the array of lasers; and an array ofmodulators, located on the emission side of the array of lasers andconnected to the drive electronics.
 2. The unit of claim 1 wherein thearray of lasers is an array of vertical cavity surface emitting lasers(VCSELs).
 3. The unit of claim 2 wherein the VCSELS are top emittingVCSELS.
 4. The unit of claim 2 wherein the VCSELS are bottom emittingVCSELS.
 5. The unit of claim 1 wherein the array of lasers is an arrayof distributed feedback (DFB) lasers.
 6. The unit of claim 5 wherein theDFB lasers further comprise elements that cause the DFB lasers to emitperpendicular to a wafer plane of the DFB lasers.
 7. The unit of claim 6wherein the elements are gratings that cause the DFB lasers to top emit.8. The unit of claim 6 wherein the elements are gratings that cause theDFB lasers to bottom emit.
 9. The unit of claim 6 wherein the elementsare micromirrors that cause the DFB lasers to top emit.
 10. The unit ofclaim 6 wherein the elements are micromirrors that cause the DFB lasersto bottom emit.
 11. The unit of claim. 1 wherein the array of lasers isan array of distributed Bragg reflector (DBR) lasers.
 12. The unit ofclaim 11 wherein the DBR lasers further comprise elements that cause theDFB lasers to emit perpendicular to a wafer plane of the DBR lasers. 13.The unit of claim 12 wherein the elements are gratings that cause theDBR lasers to top emit.
 14. The unit of claim 12 wherein the elementsare gratings that cause the DBR lasers to bottom emit.
 15. The unit ofclaim 12 wherein the elements are micromirrors that cause the DBR lasersto top emit.
 16. The unit of claim 12 wherein the elements aremicromirrors that cause the DBR lasers to bottom emit.
 17. The unit ofclaim 1 wherein the at some of the modulators are configured forexternal modulation of the beams.
 18. The unit of claim 1 wherein the atleast some of the modulators are configured as detectors.